A secondary battery has been widely used as a power source for mobile devices, such as mobile phones, laptop computers, video cameras, etc. Especially, a lithium secondary battery has been increasingly used since the lithium secondary battery has high operating voltage and high energy density per unit weight.
The lithium secondary battery mainly uses a lithium-based oxide as a cathode active material and a carbonaceous material as an anode active material. Based on kinds of electrolytic solution, the lithium secondary battery may be generally classified as a liquid electrolyte battery or a polymer electrolyte battery. Based on forms of used electrolyte, the lithium secondary battery may be also classified as a lithium-ion battery, a lithium-ion polymer battery, or a lithium polymer battery. Based on appearances of the battery, the lithium secondary battery may be classified as a cylindrical battery, a prismatic battery, or a pouch-shaped battery.
An electrode assembly constituting the secondary battery is constructed in a structure in which separators are disposed between cathodes and anodes. Typical examples of the electrode assembly include a jelly-roll type electrode assembly constructed in which long-sheet type electrodes and separators are wound in the sectional shape of a circle and a stacking type electrode assembly constructed in a structure in which electrodes and separators having a predetermined size are sequentially stacked one on another.
The electrode assembly with the above-stated construction has a problem in that short circuits may occur in the electrode assembly due to introduction of conductive foreign matter into the electrode assembly or dendrite growth at the anodes although the cathodes and anodes are electrically isolated from each other by the separators.
For this reason, a technology for attaching the ends of neighboring separators to each other by thermal welding (Japanese Unexamined Patent Publication No. 2000-090965 and Korean Unexamined Patent Publication No. 2004-0042373) and a technology for forming an insulation material at the periphery of separators to a thickness greater than ½ of the thickness of electrodes in a battery using lithium as an anode material (Japanese Unexamined Patent Publication No. 1993-109435) have been proposed to improve the safety of the battery. However, the above-mentioned conventional arts have the following several problems.
First, the technology for attaching the ends of neighboring separators to each other by thermal welding is accomplished by applying heat sufficient to cause the separators to be at least partially melted to the ends of electrodes. Consequently, electrode active materials may be damaged in the course of applying high heat so as to completely seal the separators. When the thermal welding operation is carried out at relatively low temperature in order to prevent the electrode active material from being damaged, it is difficult to accomplish complete sealability between the separators.
Second, the technology for forming an insulation material at the periphery of separators to a thickness greater than ½ of the thickness of electrodes is characterized in that the insulation material is dissolved in a volatile solvent, is injected to the ends of the separators, and is then dried. However, processes must be repeatedly carried out so as to form sealed parts having more than a predetermined size, and the drying process must be essentially carried out. For this reason, this technology is not regarded as an economic technology applicable to practical mass production.
Consequently, the demand of a technology for solving the problems of the above-mentioned conventional arts and preventing the occurrence of short circuits in the battery due to various factors is very high.